Insulation sheet and multi-layer wiring substrate and production processes thereof

ABSTRACT

The insulation sheet for use in producing a wiring substrate comprises, as via hole conductors, conductive paste filled in via holes formed through the insulation sheet, and the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet.  
     In case where a wiring substrate is produced by laminating such an insulation sheet that has the conductive paste in the via holes, and subjecting to thermo-compression bonding, the problems which conventional wiring substrates suffer from, that is, the deformation of via holes and the dislocation of the via holes because of the molten insulation sheet, which make it hard to form highly reliable electrical connections, can be solved.

CROSS-REFERENCE OF RELATED APPLICATION

[0001] The present application claims the priority under the provision of Article 119 of the U.S. Patent Law, on basis of Japanese Patent Application No. 2001-296819, entitled “Multi-Layer Wiring Substrate and Circuit Component-Including Multi-Layer Wiring Substrate, and Production Processes thereof”, filed on Sep. 27, 2001, and the content of this application is incorporated in the present specification by this citation.

FIELD OF THE INVENTION

[0002] The present invention relates to a both-sided wiring substrate (or board) in which electric connection is performed through via hole conductors provided by forming via holes through an insulating layer containing at least an organic resin, and filling the via holes with electrically conductive paste. The present invention also relates to a circuit component-including both-sided wiring substrate, a multi-layer wiring substrate and a circuit component-including multi-layer wiring substrate, and production methods of the same.

[0003] In this regard, the term “wiring substrate” used herein is already known to those skilled in the art, and various explanations are possible as to this term. For example, in the field of this art, the wiring substrate is an elements which forms a variety of parts which constitutes electronic devices, and it comprises at least one insulating layer and at least one wiring layer (or a metal wiring layer) to which a via hole conductor formed through the insulating layer is electrically connected.

RELATED ART

[0004] Recently, highly functional, small, thin and light electronic devices for mobile communication apparatuses represented by a cellular phone have been developed. In association with the development of such electronic devices, various techniques are vigorously developed so as to make it possible to provide small and thin electronic components for composing such electronic devices. Further, various techniques are also vigorously developed so as to make it possible to provide wiring substrates which permit high density mounting of these electronic components. With the rapid advance of the mounting techniques, there arise more and more demands for direct mounting of semiconductor bare chips on wiring substrates, and demands for multi-layer wiring substrates capable of accommodating to high-speed signal-processing circuits are also growing. To meet such demands, inner via hole (IVH) connection systems, which are interlayer electrical connection systems for multi-layer wiring substrates, are employed for the wiring substrates. As such wiring substrates, multi-layer wiring resin substrates which use resin materials for insulating layers, multi-layer wiring composite substrates which use mixtures of ceramic powder and resin materials for insulating layers, etc. have been provided by various manufactures.

[0005] In such multi-layer wiring substrates, the electrical connection between wiring layers is made by via hole conductors. Generally, such via hole conductors are formed by forming via holes at predetermined positions of an insulating layer which is composed of an insulation sheet formed of a thermosetting resin or an insulation sheet which contains a thermosetting resin generally as a main components; and plating the inner walls of the via holes with a metal. This process for forming the via holes, however, has problems in productivity and cost: that is, chemicals for use in the chemical plating treatment are expensive, and the treatment requires a long time. In a case where a wiring substrate having a plurality of wiring layers is produced, it is difficult to form the via hole conductors capable of performing electrical connection at any optionally predetermined positions between the wiring layers, and therefore, it is difficult to form a wiring layer having a high density wiring pattern thereon.

[0006] To solve this problem, a process is employed that comprises the steps of forming via holes at predetermined positions through an insulation sheet; filling the via holes with conductive paste which comprises an organic binder (containing a thermosetting resin as a main component) and electrically conductive powder; heating and pressing such an insulation sheet together with wiring layers, thereby heat-pressing the via holes filled with the conductive paste to compact the conductive powder in a high density and to allow the respective particles of the conductive powder to contact with one another so as to form the via hole conductors; and thereby electrically connecting the wiring layers on both sides of the insulation sheet through such via hole conductors. This process makes it possible to form the via holes at optional positions without the need of chemical treatment such as plating, to thereby make it possible to achieve the high density electrical connection in the multi-layer wiring substrates.

[0007] In a case of producing a multi-layer wiring substrate in which the electrical connection between wiring layers is performed by the via hole conductors filled with conductive paste as mentioned above, the insulation sheet having the via holes filled with the conductive paste is heated and pressed and cured together with predetermined wiring layers. Both of the thermosetting resins contained in the conductive paste and the insulation sheet are highly viscous or are practically in the form of solids at room temperatures: that is, such resins are not flowable at a room temperature. However, when heated, the thermosetting resins have a lower viscosity with rise in temperature, or are melted and converted from the solids into fluids having a very high viscosity (in this state, they are tacky). With further rise in temperature, the viscosity of the thermosetting resins further decreases, so that, the resins are not able to or almost unable to keep their forms which the resins have at a room temperature, even without any external force applied thereto (namely, naturally): that is, the resins become flowable. In this specification, since such change of the resins into becoming flowable may be referred to as apparently being melted, such apparent change is expressed by the term “melt.” When the thermosetting resins are further heated after undergoing such molten states, cure of the resins practically proceeds to increase the viscosity of the resins which have once lowered.

[0008] In this regard, the insulation sheet for forming the insulating layer may be formed of a mixture which contains, in addition to the thermosetting resin, a component which does not melt at temperature that the resin encounters in the course of producing the wiring substrate. For example, the insulation sheet may contain ceramic powder in order to improve the heat radiation of the substrate to be produced, and further may contain inorganic filler, glass fibers or the like as a reinforcement in addition to or instead of the ceramic powder. A substrate comprising an insulating layer formed of such an insulation sheet is called a composite wiring substrate. The above non-melted component is very small in size, and therefore, practically behaves together with the thermosetting resin when the thermosetting resin is melted. That is, while the thermosetting resin in the insulation sheet becomes flowable, the non-melted component can be flowable together with the thermosetting resin. In other words, a whole of the insulation sheet is in a molten state and becomes flowable.

[0009] As can be understood from the above description, the term “melt” does not mean a change from a solid into a liquid as used in the strictly scientific sense, but means that the thermosetting resin or a material for forming the insulating layer becomes able to flow.

[0010] Particularly in a case of the multi-layer wiring composite substrate which comprises an insulating layer which contains a mixture of the ceramic powder and the resin material, an insulation sheet containing no reinforcement such as glass fibers is used in order to improve the heat release of the substrate. In this case, the viscosity of the insulation sheet being melted is relatively low, as compared with an insulation sheet containing the reinforcement. In this regard, the viscosity of the insulation sheet being melted, referred to herein, means a viscosity of a whole of the materials forming the insulation sheet. For example, in a case of the insulation sheet containing a non-melted component, the viscosity which is measured under the condition wherein the insulation sheet contains such a component is intended.

[0011] Therefore, when the insulation sheet having the the via holes filled with the conductive paste is heated and pressed in the course of the process for producing the multi-layer wiring composite substrate in which performs electrical connection between the wiring layers through the above via hole conductors is formed, the insulation sheet itself melts and becomes flowable. As a result, application of a sufficient pressure is impossible because the shapes of the via holes tend to deform. Under an insufficient pressure, the conductive paste in the via holes can not be sufficiently compressed up to a high density, so that there arise a problem that highly reliable electrical connection becomes impossible, and a problem that the positions of the via holes are dislocated from their original positions at which the via holes have been formed through the insulation sheet, with the result of poor positional accuracy of the via holes. There is a further problem in that, when the insulation sheet melts and becomes flowable, the conductive paste overflows from the via holes and adhere to the surface of the insulation sheet and an unintended region other than the predetermined region between the wiring layer and the conductor, with the result that short circuit is formed by the attaching paste between the adjacent wiring conductors.

[0012] In a case of producing the wiring substrate having a circuit component (e.g., a chip component such as a bare chip semiconductor device and LCR) embedded in the insulating layer, the flowability of the insulation sheet imparted by melting of the insulation sheet is necessary for the circuit components to be embedded in the insulation sheet. When a circuit component is embedded in the insulation sheet, such an amount of the material of the insulation sheet that corresponds to the volume of the circuit component is push away, so that the material moves. The foregoing problem in association with the flowing of the insulation sheet itself, therefore, becomes more serious. Therefore, it is very difficult to achieve at the same time both objects: that is, to achieve high density mounting of circuit components by embedding the circuit components in the insulation sheet, and to accomplish high density wiring by the shortest electrical connection through the inner via holes filled with conductive paste.

SUMMARY OF THE INVENTION

[0013] An object of the present invention is therefore to provide a both-sided wiring substrate, a circuit component-embedded both-sided wiring substrate, a multi-layer wiring substrate and a circuit component-embedded multi-layer wiring substrate, and processes for producing the same.

[0014] One aspect of the present invention is to provide an insulation sheet for use in producing a variety of wiring substrates, and this insulation sheet comprises a via hole(s) which is formed at predetermined position(s) through the insulation sheet and filled with electrically conductive paste which has a curing-starting temperature lower than a melting-starting temperature of the insulation sheet. The insulation sheet forms an insulating layer in the wiring substrate, and for example a via hole conductor(s) formed through the via hole(s) which pass(es) through the insulating layer electrically connects wiring layers on both sides of the insulating layer to each other at a predetermined position(s). An electronic component(s) and the like is mounted on the produced wiring substrate, as required, and an electronic device is produced, using the resultant wiring substrate.

[0015] Another aspect of the present invention is to provide a process for producing the above insulation sheet, and the process comprises forming a predetermined via hole(s) through an insulation sheet, and filling the via hole(s) with a conductive paste which has a curing-starting temperature lower than a melting-starting temperature of the insulation sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a sectional view of a multi-layer wiring substrate according to one embodiment of the present invention.

[0017]FIG. 2 is a sectional view of a circuit component-embedded multi-layer wiring substrate according to one embodiment of the present invention.

[0018]FIG. 3 shows sectional views illustrating the steps for producing a multi-layer wiring substrate according to one embodiment of the present invention.

[0019]FIG. 4 shows sectional views illustrating the steps for producing a circuit component-embedded multi-layer wiring substrate according to one embodiment of the present invention.

[0020]FIG. 5 shows sectional views illustrating the steps for producing a circuit component-embedded multi-layer wiring substrate according to another embodiment of the present invention.

[0021]FIG. 6 shows a DTA curve indicating the curing-starting temperature of conductive paste.

[0022] In these drawings, the reference numerals represents the following elements:

[0023]100=insulating layer, 101 via hole conductor,

[0024]102=wiring layer, 110=multi-layer wiring substrate,

[0025]200=insulating layer, 201=via hole conductor,

[0026]202 metal wiring, 203=circuit component,

[0027]210=circuit component-including multi-layer wiring substrate,

[0028]300=insulation sheet, 301=via hole, 302=via hole conductor,

[0029]303=wiring layer (or wiring), 304=release carrier,

[0030]310=insulation sheet, 315=transferable wiring pattern-including member,

[0031]320, 330 or 340=insulation sheet,

[0032]350 multi-layer wiring substrate,

[0033]400=insulation sheet, 401=via hole,

[0034]402=via hole conductor,

[0035]403=wiring layer (or wiring),

[0036]404=release carrier, 405=circuit component,

[0037]410 insulation sheet,

[0038]415 or 416=transferable wiring pattern-including member,

[0039]420, 430 or 440=insulation sheet,

[0040]450=circuit component-including multi-layer wiring substrate,

[0041]500=insulation sheet, 501=via hole,

[0042]502=via hole conductor formed by filling with conductive paste,

[0043]503=metal foil, 504=wiring layer (or wiring),

[0044]505=circuit component, 510, 520 or 530=insulation sheet,

[0045]540=metal foil, and 550=circuit component-including multi-layer wiring substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0046] In the foregoing aspects of the present invention (i.e., the insulation sheet and the process for producing the same), the conductive paste comprises electrically conductive powder (e.g., copper particles) and an organic binder. The organic binder contains a thermosetting resin (e.g., an epoxy resin), and may further contain a curing agent and/or a curing accelerator. The conductive paste may contain an appropriate organic solvent (e.g., butyl carbitol) in order to adjust the viscosity of the paste. In the above aspects of the present invention, the conductive paste is in such a paste condition as to be filled in the via holes, for example, by screen printing, and the thermosetting resin contained in the paste is not yet cured when the paste is filled in the via holes.

[0047] In the present invention, the insulation sheet contains a thermosetting resin, and if needed, other component. For example, the insulation sheet may contain ceramic powder or the like in order to accelerate heat radiation, and glass fibers or the like in order to reinforce the sheet. The conductive paste and the insulation sheet to be used in the present invention may be of the same kind or of different kinds, in so far as the above relationship between the curing-starting temperature of the conductive paste and the melting-starting temperature of the insulation sheet is satisfied. Further, known conductive paste and known insulation sheets used for producing the wiring substrates by means of the via hole connection system may be used. In the above aspects of the present invention, the insulation sheet is a member in the form of a sheet, and the thermosetting resin contained in the insulation sheet is in an uncured state.

[0048] In another aspect, the present invention provides a process for producing a wiring substrate, in which via hole conductors formed by filling predetermined via holes with the conductive paste are electrically connected to a predetermined wiring layer disposed on at least one of the main surfaces (which define the layer and are present on both sides of the layer) of the insulating layer. This process includes the step of electrically connecting, to the wiring layer disposed on the insulation sheet, the via hole conductors which are formed by filling the via holes with the conductive paste which contains a thermosetting resin and has the curing-starting temperature lower than the melting-starting temperature of the insulation sheet forming the insulating layer. This process for producing the wiring substrate comprises forming the insulation sheet of the present invention as described above, then disposing the wiring layer on at least one of the main surfaces of the insulation sheet, and electrically connecting, to the wiring layer, the via hole conductors formed of the conductive paste. In this regard, upon electrically connecting the via hole conductors to the wiring layer disposed on the insulation sheet, the electrically connected portions of the wiring layer are positionally aligned relative to the via holes formed through the insulation sheet.

[0049] Therefore, the process for producing a wiring substrate of the present invention comprises:

[0050] (a) preparing the foregoing insulation sheet of the present invention (in which the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet), and

[0051] (b) disposing a wiring layer on at least one of the main surfaces of the insulating layer, and electrically connecting the via hole conductors to the wiring layer. In this production process, a variety of wiring substrates can be produced thereafter through steps which will be described later.

[0052] In the present invention, “disposing the wiring layer and electrically connecting the via hole conductors to the wiring layer” may be carried out by any manner in so far as the connection can be so formed that current can flow in a predetermined manner between the via hole conductors formed by filling predetermined via holes with the conductive paste and the predetermined wiring layer, and thus, predetermined regions of the wirings which form composing the wiring layer are connected to the via hole conductors. Other regions than the predetermined regions of the wiring layer are bonded to the insulation sheet, and current, of course, does not flow in such regions.

[0053] In one of preferred embodiments, the predetermined wiring layer is disposed on the insulation sheet having the via holes filled with the conductive paste, and the wiring layer and the insulation sheet are pressed together while being heated (thermal-pressing) to thereby achieve the electrical connection. In this embodiment, the predetermined wiring layer composed of various predetermined wirings may be thermally-transferred to the insulation sheet. In a case of the thermal transfer, a transferable wiring pattern-including member comprising a release carrier having a transferable metal wiring layer on its surface is thermally-pressed to transfer the metal wiring layer onto the insulation sheet, and thus, the wiring layer is formed on the insulation sheet.

[0054] In case of employing the thermal transfer, it is possible to previously form fine metal wirings on a release carrier. In this regard, the transferable wiring pattern-including members may have a structure in which a transferable wiring layer is directly formed on a release carrier, or a structure in which a transferable wiring layer is formed on the release carrier through a very thin release layer. The release carrier is generally formed of a metal or a resin. The transferable wiring patter-including member may have an electrically connected circuit component on the transferable metal wiring layer. Such a transferable wiring pattern-including member is used to embed the circuit component in the insulation sheet when the wiring layer is electrically connected to the main surface of the insulation sheet, as described later. In this case, the circuit component can be embedded in the insulating layer simultaneously with the arrangement and electrical connection of the metal wiring layer.

[0055] In another preferred embodiment, a metal foil is placed on an insulation sheet having the via holes filled with conductive paste, and they are bonded together by heating and if needed, pressing them (thermal bonding), and then, the metal foil is, for example, chemically etched by the subtractive method so that only necessary regions as a predetermined wiring layer can be left to remain, and thereby, the metal foil is electrically connected to the insulation sheet. In case of employing the subtractive method such as chemical etching, less expensive metal wirings can be formed, using a metal wiring-forming etching apparatus for the conventional wiring substrates as it is, particularly when a copper foil is used as the metal foil for forming the wiring layer.

[0056] In any of the above embodiments, the electrical connection between the via hole conductors and the wiring layer can be ensured, because the thermosetting resin in the binder is heated to have tackiness and further because the conductive paste in the via hole conductors is compressed to density the conductive powder. In addition, also, the thermosetting resin in the insulation sheet is heated to have tackiness, whereby contributing to the bonding between the wiring layer and the insulation sheet. Therefore, the step of electrically connecting the wiring layer disposed as mentioned above is a step of thermo-compression bonding the wiring layer to the via hole conductors, and specifically a step of bonding predetermined regions of the wiring layer to the via hole conductors, and bonding other regions of the wiring layer to the insulation sheet. In the process for producing the wiring substrate of the present invention, the predetermined wiring layer (generally a metal wiring layer) is an assembly of predetermined wirings in the form of a thin layer as a whole. The wiring layer is to finally form the predetermined wirings on the insulating layer, and these wirings are disposed on one or both sides of the insulation sheet together in the form of layer(s).

[0057] In the process for producing a wiring substrate of the present invention, because of the feature of the curing-starting temperature of the conductive paste lower than the melting-starting temperature of the insulation sheet, the conductive paste in the via holes is cured in advance by the thermo-compression bonding prior to the melting of the insulation sheet. Therefore, the curing of the conductive paste already has proceeded at a point of time when the insulation sheet is so heated as to melt. Preferably, the curing of the conductive paste has considerably proceeded at such a point of time, or more preferably, the curing of the conductive paste has been practically completed at such a point of time. In one of the embodiments, the thermo-compression bonding is carried out in a plurality of steps: that is, in the first step, the conductive paste is completely cured without practically melting the insulation sheet, and then, the insulation sheet is melted and then completely cured in at least one of the following steps.

[0058] As a result, the following effect is produced:

[0059] Even if the applied heat finally melts the insulation sheet, an adverse influence of becoming flowable of the insulation sheet on the via hole conductors can be suppressed minimum.

[0060] (A) in the course of the thermo-compression bonding carried out for the above electrical connection, or

[0061] (B) as will be described later,

[0062] (a) when a both-sided circuit substrate or a single-sided circuit substrate is produced by continuing the thermo-compression bonding carried out for the electrical connection under conditions which are the same as or different from those upon the thermo-compression bonding after the thermo-compression boding carried out for the electrical connection, or

[0063] (b) when a multi-layer wiring circuit substrate is produced by further carrying out the thermo-compression bonding together with at least one other insulting sheet (corresponding to a third insulation sheet as described later) after the thermo-compression boding carried out for the above electrical connection.

[0064] To distinct the thermo-compression bonding which is further carried out in the above case (a) or (b), that is, the thermo-compression bonding carried out after the thermo-compression bonding for electrical connection is referred to as “post-thermo-compression bonding” in the present specification.

[0065] Particularly when the insulating layer obtained by electrically connecting the via hole conductors to the wiring layer is thereafter subjected to the post-thermo-compression bonding, the shapes of the via hole conductors are hard to deform, and thus, the influence of becoming flowable of the insulation sheet can be advantageously avoided. Particularly in a case where the curing of the thermosetting resin in the binder already has sufficiently proceeded in the course of the electrical connection, the conductive paste in the via holes has become hard. Therefore, no substantial change in the shapes of the via hole conductors is caused.

[0066] In the above process for producing the wiring substrate, as mentioned above, it is preferable that curing of the thermosetting resin in the binder practically proceeds in the course of the thermo-compression bonding for electrically connecting the via hole conductors to the wiring layer, and more preferable that the curing of the thermosetting resin in the binder has been practically completed in the course of the same thermo-compression-bonding. In this course of the thermo-compression bonding, curing of the thermosetting resin in the insulation sheet may practically proceed or may not proceed. However, this thermosetting resin is not allowed to be completely cured. For example, the insulation sheet may be in a semi-cured condition by the thermo-compression bonding, and such semi-cured condition is preferable in some cases. Those skilled in the art could select the curing state of the thermosetting resin in the insulation sheet after subjected to the electrical connection, in accordance with how to use the insulation sheet which has undergone the electrical connection.

[0067] In one of the embodiments of the above process for producing the wiring substrate, (c) a wiring substrate is obtained by further subjecting only the insulation sheet which has undergone the electrical connection (referred to as a first insulation sheet for clearly distinguishing) to the post-thermo-compression bonding, thereby completing the curing of the thermosetting resin in the insulation sheet for obtaining the wiring substrate. In this case, a single-sided wiring substrate or a both-sided wiring substrate can be obtained when wiring layer(s) is or are disposed on one or both sides of the insulation sheet respectively and electrically connected.

[0068] In this case, the post-thermo-compression bonding may be carried out by continuing the thermo-compression bonding under the same conditions as in the first thermo-compression bonding for the electrical connection. In other words, the first thermo-compression bonding is continued until the curing of the insulation sheet is completed to thereby carry out the post-thermo-compression bonding. Needless to say, heating and pressing may be carried out in two or more steps under the conditions for the post-thermo-compression bonding, which are different from the conditions for the first thermo-compression bonding. For example, in the first thermo-compression bonding, only the conductive paste is sufficiently cured, and in the post-thermo-compression bonding, the insulation sheet alone is substantially cured.

[0069] At least one other insulation sheet (referred to as a second insulation sheet for clearly distinguishing) which is incompletely cured is aligned relative to and laminated on at least one of the main surfaces of the above single-sided or both-sided wiring substrate, and subjected to thermo-compression bonding treatment (the word “treatment” is added to clearly distinguish from “the thermo-compression bonding” which the insulating layer firstly undergoes and “the post-thermo-compression bonding” which the insulating layer undergoes thereafter) to thereby be bonded together to obtain a multi-layer wiring substrate. This second insulation sheet is not particularly limited in selection of the-type and the number of sheets, in so far as a desired multi-layer wiring substrate can be obtained by laminating this insulation sheet on the single-sided wiring substrate or the both-sided wiring substrate, and then subjecting the resultant laminate to the thermo-compression bonding treatment. In this connection, a plurality of the second insulation sheets may be laminated is on the wiring substrate. Generally, the second insulation sheet has via hole conductors formed by filling via holes with a conductive paste, and may have a wiring layer electrically connected to the via hole conductors, on at least one of the main surfaces of the insulating layer. Therefore, the second insulation sheet may have no wiring layer on either side thereof, while, however, if needed, the second insulation sheet may have wiring layer(s) on one side or both sides thereof. The number of the second insulation sheets and the kind thereof are selected so that all the wiring layers can be electrically connected in a predetermined manner when the second insulation sheet(s) together with the single-sided wiring substrate or the both-sided wiring substrate are subjected to the thermo-compression bonding treatment to obtain the multi-layer wiring substrate. In the above single-sided or both-sided wiring substrate of the present invention, the curing of the insulation sheet has been completed, and therefore, the second insulation sheet is not completely cured, and preferably, it is uncured or semi-cured.

[0070] Concretely, the above process for producing the multi-layer wiring substrate comprises:

[0071] (d) laminating at least one second insulation sheet which has via hole conductors, and if needed, a wiring layer electrically connected to the via hole conductors, on at least one of the main surfaces thereof, on at least one of the main surfaces of the above single-sided or both-sided wiring substrate,

[0072] (e) upon such lamination,

[0073] (e-1) aligning the single-sided or both-sided wiring substrate relative to the second insulation sheet to be laminated thereon, so that (i) the predetermined regions of the wiring layer of the single-sided or both-sided wiring substrate can be electrically connected to the via hole conductors of the second insulation sheet in a predetermined manner by the post-thermo-compression bonding treatment which will be subsequently carried out, or that (ii) the via hole conductors of the single-sided or both-sided wiring substrate can be electrically connected to the predetermined regions of the wiring layer of the second insulation sheet adjacent thereto in a predetermined manner by the thermo-compression bonding treatment which will be subsequently carried out; and

[0074] (e-2) aligning the remaining other second insulation sheet so that the second insulation sheets can be electrically connected to each other in a predetermined manner by subsequent thermo-compression bonding treatment which will be subsequently carried out; and

[0075] (f) subjecting the laminate of the single-sided or both-sided wiring substrate and the second insulation sheet(s) to the thermo-compression bonding treatment to obtain the multi-layer wiring substrate.

[0076] In the above (e-1), the item (i) corresponds to a case where the main surface of the-first insulation sheet which opposite to the second insulation sheet has the wiring layer thereon, and in such a case, the main surface of the second insulation sheet which is opposite to the first insulation sheet has no wiring layer. Thus, the wiring layer of the first insulation sheet is electrically connected to the via hole conductors of the second insulation sheet by the thermo-compression bonding.

[0077] In the above (e-1), the item (ii) corresponds to a case where the main surface of the first insulation sheet which is opposite to the second insulation sheet has no wiring layer, and in such a case, the main surface of the second insulation sheet which is opposite to the first insulation sheet has the wiring layer thereon. Thus, the via hole conductors of the first insulation sheet are electrically connected to the wiring layer of the second insulation sheet by the thermo-compression bonding.

[0078] The above (e-2) is carried out in a case where a plurality of the second insulation sheets are laminated and the second insulation sheet which is not directly adjacent to the single-sided or both-sided wiring substrate must be also electrically connected in a predetermined manner.

[0079] In another embodiment of the process for producing the wiring substrate of the present invention, at least one other insulation sheet (referred to as a third insulation sheet for distinguishing from the above second insulation sheet) is laminated on the insulation sheet obtained after undergoing the electrical connection (i.e., after the thermo-compression bonding) (also, referred to as a first insulation sheet), and these insulation sheets are integrally subjected to post-thermo-compression bonding to thereby produce the multi-layer wiring substrate. The third insulation sheet may be the same as the above mentioned second insulation sheet except that curing state of the third insulation sheet (specifically the curing state of the thermosetting resin in the third insulation sheet) may be in any state. Therefore, the foregoing description as to the second insulation sheet can be applied to the third insulation sheet except for the curing state thereof.

[0080] The third insulation sheet contains a thermosetting resin, and has through via holes at predetermined positions which are filled with conductive paste to form via hole conductors. The third insulation sheet may have a predetermined wiring layer on at least one side thereof as required, and the above via hole conductors formed by filling the via holes with the conductive paste are electrically connected to the wiring layer. Therefore, the third insulation sheet may have predetermined wiring layers on both main surfaces thereof (these wiring layers being electrically connected to the via hole conductors) or may have a predetermined wiring layer on one of the main surfaces thereof (this wiring layer being electrically connected to the via hole conductors), or may have no wiring layer thereon.

[0081] In a case where the multi-layer wiring substrate is produced, curing of the first insulation sheet obtained by the process of the present invention has not been yet completed after the electrical connection (i.e., uncured or semi-cured), and therefore, the first insulation sheet can be laminated on the third insulation sheet and subjected to thermo-compression bonding integrally, even though the third insulation sheet is in any curing state. Generally, it is preferable that the third insulation sheet is cured or semi-cured.

[0082] The above process for producing the multi-layer wiring substrate comprises:

[0083] (D) laminating at least one third insulation sheet which has the via hole conductors and if needed, the wiring layer electrically connected to the via hole conductor on at least one of the main surfaces thereof, on at least one of the main surfaces of the first insulation sheet obtained after the thermo-compression bonding,

[0084] (E) upon such lamination,

[0085] (E-1) aligning the first insulation sheet relative to the third insulation sheet to be laminated thereon, so that (i) the predetermined regions of the wiring layer of the first insulation sheet can be electrically connected to the via hole conductors of the second insulation sheet to be laminated thereon in a predetermined manner by the post-thermo-compression bonding which will be subsequently carried out; or so that (ii) the via hole conductors of the first insulation sheet can be electrically connected to the predetermined regions of the wiring layer of other insulation sheet adjacent to the first insulation sheet in a predetermined manner by the post-thermo-compression bonding which will be subsequently carried out; and

[0086] (E-2) aligning the remaining other second insulation sheet so that the second insulation sheets can be electrically connected to each other in a predetermined manner by the post-thermo-compression bonding which will be subsequently carried out; and

[0087] (F) subjecting the resultant laminate of the first insulation sheet and the second insulation sheet(s) to the post-thermo-compression bonding to obtain the multi-layer wiring substrate.

[0088] In the above (E-1), the item (i) corresponds to a case where the main surface of the first insulation sheet which is opposite to the third insulation sheet has the wiring layer thereon, and in such a case, the main surface of the third insulation sheet which is opposite to the first insulation sheet has no wiring layer. Thus, the wiring layer of the first insulation sheet is electrically connected to the via hole conductors of the third insulation sheet by the post-thermo-compression bonding.

[0089] In the above (E-1), the item (ii) corresponds to case where the main surface of the first insulation sheet which is opposite to the third insulation sheet has no wiring layer, and in such a case, the main surface of the third insulation sheet which is opposite to the first insulation sheet has the wiring layer thereon. Thus, the via hole conductors of the first insulation sheet is electrically connected to the wiring layer of the third insulation sheet by the post-thermo-compression bonding.

[0090] The above (E-2) is carried out in a case where a plurality of the third insulation sheets are laminated and the insulation sheet which is not directly adjacent to the single-sided or both-sided wiring substrate must be electrically connected in a predetermined manner.

[0091] Accordingly, the above process for producing the wiring substrate of the present invention includes laminating the insulation sheet obtained after undergoing the electrical connection of the via hole conductor to the wiring layer, that is, any one of the foregoing various first insulation sheets, on at lest one other insulation sheet, that is, the third insulation sheet; and further subjecting the laminate of these insulation sheets to the thermo-compression bonding, i.e., to the post-thermo-compression bonding to thereby integrally cure the fist insulation sheet and the third insulation sheet to obtain the wiring substrate, i.e., a multi-layer wiring substrate.

[0092] In a case where the first insulation sheet laminated on the third insulation sheet is subjected to the post-thermo-compression bonding, needless to say, the first insulation sheet and the third insulation sheet are aligned relative to each other so that the wiring layers of the first insulation sheet and the third insulation sheet to be laminated on each other can be electrically connected to each other through the via hole conductors in a predetermined manner, and then subjected to the post-thermo-compression bonding.

[0093] In this regard, the above post-thermo-compression bonding may be carried out a plurality of steps. In case where the conductive paste has not been completely cured, the conductive paste is completely cured in the first step without practically melting the insulation sheet or without allowing curing of the insulation sheet to practically proceed, and then, the insulation sheet is melted in at least one of the following steps, and then completely cured.

[0094] The second or third insulation sheet laminated as above is preferably an insulation sheet which the same one as the insulation sheet electrically connected according to the foregoing process for producing the wiring substrate of the present invention (that is, the first insulation sheet). That is, it is preferable that the curing-starting temperature of the conductive paste of the second or third insulation sheet is lower than the melting-starting temperature of the insulation sheet. If needed, such a third insulation sheet may have a circuit component embedded therein as described below.

[0095] Alternatively, the second or third insulation sheet may be a separately produced insulation sheet which is so designed as to be laminated on the first insulation sheet obtained after undergoing the electrical connection by the foregoing process of the present invention, and to be subjected to the thermo-compression bonding or the post-thermo-compression bonding. In a further embodiment, the second or third insulation sheet may be an insulation sheet which has a plurality of insulating layers and a multi-layer wiring having predetermined wiring layers each disposed between the two insulating layers. It is possible to appropriately select the kind of the third insulation sheet and the number of other insulation sheets in accordance with the wiring substrate which is intended be produced.

[0096] In the process for producing the wiring substrate of the present invention, when the thermo-compression bonding carried out for the electrical connection is such that the insulation sheet is not caused to melt, the insulation sheet is melted by the post-thermo-compression bonding, and then, curing of the thermo-curable resin proceeds. Also, when the thermo-compression bonding carried out for the electrical connection is such that the insulation sheet is caused to melt, curing of the insulation sheet is allowed to further proceed by the post-thermo-compression bonding up to a sufficient degree. When curing of the conductive paste is not sufficiently proceed by the thermo-compression bonding carried out for the electrical connection, curing of the conductive paste is allowed to further proceed by the post-thermo-compression bonding up to a sufficient degree. Because of the feature of the present invention that the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet, curing of the conductive paste has already started before the insulation sheet starts melting. Therefore, the problems in association with melting of the insulation sheet are at least alleviated.

[0097] In one of the preferred embodiments of the process for producing the wiring substrate of the present invention, the wiring layer which is electrically connected to the via hole conductors formed inside the via holes provided through the insulation sheet (i.e., the wiring layer disposed on the first insulation sheet) may have at least one circuit component which is electrically connected to the wiring layer in advance (for example, a bare chip semiconductor device, a chip functioning as a resister, a chip functioning as a capacitor, a chip functioning as an inductor, an LCR chip or the like). In this embodiment, when the wiring layer is disposed on the insulation sheet and electrically connected to the via hole conductors, the wiring layer is exposed on the surface of the insulation sheet and the circuit component is embedded in the insulation sheet. Concretely, the wiring layer is thermally-transferred to the insulation sheet as follows: a transferable wiring layer support carrier, which has thereon a wiring layer having a circuit component electrically connected thereto in advance, is so disposed on the insulation sheet that the wiring layer can be exposed after the thermal transfer, and then subjected to the thermo-compression bonding; and thereafter, the carrier is released. Therefore, according to this embodiment of the process, when the wiring layer on at least one of the main surfaces of the insulation sheet is electrically connected to the via hole conductors of the insulation sheet, the circuit component is embedded in the insulation sheet, and the wiring layer electrically connected to the circuit component is exposed on the surface of the insulation sheet. In this embodiment, it is necessary for the thermo-compression bonding to be carried out under a condition which allows the insulation sheet to melt, in order to embed the circuit component in the insulation sheet, the curing of the insulation sheet may proceed or not, but the complete curing of the insulation sheet is not permitted. Even, in this case, the curing of the conductive paste has already started and proceeded before the insulation sheet starts melting because of the foregoing features of the present invention. Therefore, the adverse influence caused by the melting of the insulation sheet is alleviated.

[0098] The insulation sheet having the circuit component embedded therein as above (corresponding to the first insulation sheet) is substantially the same as the foregoing insulation sheet (i.e., the first insulation sheet), except for the different point in that the circuit component is electrically connected to the wiring layer in advance (which requires the insulation sheet to be melted, so as to embed the circuit component in the insulation sheet). Accordingly, the foregoing explanation can be applied to the insulation sheet having the circuit component embedded therein except for this different point. Thus, the insulation sheet having the circuit component embedded therein can be used for the production of a both-sided or single-sided wiring substrate, or a multi-layer wiring substrate. For example, this insulation sheet is combined with other insulation sheet, i.e., the foregoing second or third insulation sheet and laminated thereon, and subjected to the thermo-compression bonding or the post-thermo-compression bonding. Thus, the multi-layer wiring substrate having the circuit component embedded therein can be provided.

[0099] In a further embodiment of the process of the present invention, at least one circuit component is electrically connected to the wiring layer of the insulation sheet having the via hole conductors which electrically connected to the wiring layer, that is the first insulation sheet (the circuit component is therefore located outside the insulation sheet, in other words, exposed to the outside), and this first insulation sheet having the circuit component electrically connected is laminated on at least one other insulation sheet (which may be the third insulation sheet as described above) and together subjected to the post-thermo-compression bonding. Thus, the multi-layer wiring substrate having the circuit component embedded therein can be provided. In this embodiment, the insulation sheet is melted by the post-thermo-compression bonding to thereby allow the circuit component to be embedded therein.

[0100] In any of the above embodiment of the present invention, the via hole conductors, formed of the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet which forms an insulating layer and contains a thermosetting resin, are electrically connected to the wiring layer disposed on the insulation sheet. Therefore, the conductive paste has been cured, preferably sufficiently cured, before the insulation sheet melts and flows. Thus, the shapes of the via hole conductors are prevented from deforming because of becoming the flowable of the insulation sheet, and it becomes possible to sufficiently compress the via hole conductors formed by filling with the conductive paste. As a result, a single-sided wiring substrate, a both-sided wiring substrate, a multi-layer wiring substrate, and these wiring substrates each having a circuit component embedded therein, in all of which highly reliable electrical connections are achieved, can be provided. Therefore, the present invention also provides the single-sided wiring substrate, the both-sided wiring substrate and the multi-layer wiring substrate which are all produced by the foregoing processes and which may optionally include the circuit components.

[0101] In the present invention, the melting-starting temperature of the insulation sheet is defined as a temperature at which a viscosity of the insulation sheet found at a predetermined temperature decreases to a certain rate with temperature rising. According to the present invention, the melting-starting temperature is a temperature at which the viscosity at a room temperature (about 30° C.) decreases to one-hundredth ({fraction (1/100)}). As a result of further intensive researches, it has been discovered to be more preferable that the melting-starting temperature is determined as a temperature at which the viscosity at 50° C. decreases to one-fiftieth ({fraction (1/50)}). When a change of the viscosity of the insulation sheet is measured, a flow tester (Shimadzu Flow Tester CFT-500D manufactured by Shimadzu Corporation) as is used an instrument.

[0102] In the present invention, the curing-starting temperature of the conductive paste is defined as follows, with reference to FIG. 6:

[0103] The conductive paste is subjected to the differential thermal analysis (heated at a rate of 5° C./min. from 30° C. to 250° C. (equivalent to a temperature not lower than the curing-starting temperature) to obtain a differential thermal curve (temperature (T, axis of abscissa) vs difference in temperature (ΔT, axis of ordinate) as diagrammatically shown in FIG. 6. Regarding the portion indicating the peak of the curve (the lowermost peak seen in FIG. 6), a decreasing point (B) of the curve (from which point, the difference in temperature (ΔT) abruptly becomes larger) is determined. Next, the curing-starting temperature is defined as a temperature which is found at the intersection (G) of an extrapolated tangent line (a) drawn at the maximal gradient point (E) within the region from the point (B) to the peak point (C) and an extrapolated tangent line (b) drawn from the point (B). This differential thermal curve is obtained, using a thermal analysis instrument (a differential thermo/thermogravity-co-measuring instrument TG/DTA200 manufactured by Seiko Instruments Inc.), and the above curing-starting temperature can be easily calculated by a software combined in the instrument.

[0104] According to the present invention, the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet, and preferably at least 5° C., more preferably at least 10° C. lower than the melting-starting temperature of the insulation sheet. The larger this difference in temperature, the more preferable. However, if this difference in temperature is too large, there arises a disadvantage in that the temperature of post-cure is too high. In one of the embodiments, the curing-starting temperature of the conductive paste is, for example, 8 to 15° C. lower than the melting-starting temperature of the insulation sheet. Those skilled in the art can appropriately set the curing-starting temperature of the conductive paste and the melting-starting temperature of the insulation sheet by selecting the kinds of the components forming these materials, and their compositions.

[0105] Embodiments for Carrying Out the Invention

[0106] In one of the preferred embodiments, the multi-layer wiring substrate of the present invention comprises at least two insulating layers, each formed of an insulation sheet of a mixture containing an inorganic filler (preferably 50 to 75 vol. % or 70 to 95 wt. % based on the volume or the weight of the mixture) and a thermosetting resin; wiring layers (generally metal wiring layers) two of which sandwich each of the insulating layers respectively; and via hole conductors formed passing through the insulating layers so as to be electrically connected to the wiring layers. The via hole conductors are formed by filling the via holes with the conductive paste which contains at least conductive powder and an organic binder, and starts curing at a temperature lower than the melting-starting temperature of the insulation sheets.

[0107] The via hole conductors, formed of the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet, can effectively maintain their shapes against melting and flowing of the insulation sheet, because the conductive paste has started curing before the insulation sheet starts melting. Thus, it becomes possible to sufficiently compress the via hole conductors to thereby compact the conductive powder in the conductive paste in a high density. Therefore, a multi-layer wiring substrate which has low resistance and highly reliable electrical connection therein can be provided. Further, since the shapes of the via hole conductors can be maintained, the via hole conductors are not dislocated by flowing of the molten insulation sheet, and thus, the positional accuracy of the via hole conductors is improved.

[0108] In the present invention, it is preferable that the curing-starting temperature of the conductive paste is in the range of 50 to 150° C. It is generally required for the conductive paste to have a viscosity sufficient to be filled in the via holes, and specifically to maintain a viscosity of 10 to 500 Pa.s at a room temperature for at least a half day. If the curing-starting temperature is lower than 50° C., the conductive paste is hard to maintain the stock stability at a room temperature. On the other hand, if higher than 150° C., the conductive paste can not be sufficiently cured before the insulation sheet starts melting which contains a mixture of, for example, an inorganic filler and a thermosetting resin. As a result, it can not be expected that the shapes of the via hole conductors, formed by filling with the conductive paste, are sufficiently maintained while the insulation sheet becomes flowable. It is particularly preferable that the curing-starting temperature of the conductive paste is 70 to 120° C. Further preferably, the curing-starting temperature of the conductive paste is 80 to 100° C.

[0109] In the present invention, the electrically conductive paste contains 80 to 97 wt. %, preferably 85 to 95 wt. % of electrically conductive powder (or electrically conductive particles), and 3 to 20 wt. %, preferably 5 to 15 wt. % of an organic binder. One example of the conductive paste contains 80 to 95 wt. % of the conductive powder and 5 to 20 wt. % of the organic binder. If the amount of the conductive powder is larger than 97 wt. %, it becomes hard to form the conductive powder into the paste form. On the contrary, if smaller than 80 wt. %, a large amount of the organic binder remains in the via hole conductors, when the via hole conductors formed by filling with the conductive paste are cured. As a result, the individual particles of the conductive powder are hindered from contacting with one another, which leads to higher electrical resistance of the via hole conductors. The conductive paste may further contain a suitable amount of an organic solvent for adjusting the viscosity, such as butyl carbitol, butyl carbitol acetate or the like.

[0110] The conductive powder in the conductive paste is the powder of at least one metal selected from the group consisting of gold, silver, palladium, copper, nickel, tin, lead, and alloys thereof. Alternatively, the conductive powder may be a mass of particles each of which comprises a core of a metal selected from the above group, and a coating of other metal selected therefrom. In view of the electrical conductivity, silver is superior, and the use of silver is advantageous for the via hole conductors to have lower resistance. In a case where conductive powder of a soft metal is used, the particles of the conductive powder can have increased contact areas therebetween and therefore provide lower resistance when the via hole conductors formed by filling with the conductive paste are compressed. Therefore, gold and silver are suitable, and copper is superior in view of the cost. Further suitable conductive powder is a mass of particles each of which comprises a core of copper and a coating of silver, because this powder has both merits of copper and silver.

[0111] In the present invention, preferably, the organic binder in the conductive paste contains at least one selected from thermosetting resins such as an epoxy resin, a phenol resin, a cyanate resin and a polyphenylene ether resin. A mixture of an epoxy resin and a phenol resin is more preferable as the thermosetting resin, and the mixing ratio of the epoxy resin to the phenol resin (weight ratio) is preferably 1:4 to 4:1. This is because the use of the epoxy resin alone is effective to obtain a binder having excellent electrical insulation, however, such a binder has no function to reduce the oxidized conductive powder (metal powder) in the conductive paste, and because the use of the phenol resin alone is effective to obtain a binder excellent in reducing ability, however, such a binder has defects in that the resin itself is brittle and must be dissolved in a solvent since it is generally in the form of a solid. The epoxy resin to be used may be one of known resins such as glycidyl ether type epoxy resins, alicyclic epoxy resins, and glycidylamine type epoxy resins (e.g., Bisphenol A type, Bisphenol F type, Novolac type, Lesol type, etc.).

[0112] In the present invention, the organic binder contains preferably a curing catalyst or a curing accelerator which is composed of at least one compound selected from the group consisting of amine compounds, imidazoles, imidazole derivatives and onium salts. It is suitable to use an imidazole which is in the form of a solid at a room temperature in view of the stock stability. Such an imidazole is dissolved in an organic resin when heated to a predetermined temperature or higher, and quickly accelerates curing of the conductive paste. An onium salt type catalyst or accelerator is low in activity at a room temperature, and thus excellent in the stock stability. Further, to improve the response to a temperature, it is effective to increase the concentration of the curing agent or the curing catalyst in the resin. For example, in a case of using an imidazole as the curing catalyst, the amount of the imidazole to be added for curing the thermosetting resin is normally 0.5 to 2 parts by weight, preferably 3 to 20 parts by weight, and more preferably 5 to 15 parts by weight (based on 100 parts by weight of the thermosetting resin). This amount of the imidazole is effective to make it sure to start the curing of the conductive paste at a temperature lower than the melting-starting temperature of the insulation sheet, and also to quicken the curing of the conductive paste and improve the stock stability of the conductive paste.

[0113] In the present invention, the thermosetting resin contained in the insulation sheet preferably comprises at least one thermosetting resin selected from epoxy resins, phenol resins and cyanate resins. By using such a resin selected from the above, a wiring substrate excellent in its heat resistance and electrical insulation can be provided.

[0114] In the present invention, the insulation sheet contains preferably an inorganic filler which is preferably at least one selected from Al₂O₃, SiO₂, MgO, BN and AlN. The insulation sheet can have an excellent heat radiation property by containing such an inorganic filler. In a case of using SiO₂ as the inorganic filler, the resultant substrate can have a small dielectric constant. The insulation sheet may further contain a reinforcement such as glass fibers or the like. The insulation sheet, if needed, may contain other components such as a coupling agent for improving the dispersibility of the inorganic filler in the resin, carbon black for coloring the resin, etc.

[0115] In one of the embodiment of the process for producing the multi-layer wiring substrate of the present invention, there is produced a multi-layer wiring substrate in which metal wiring layers are electrically connected to each other through an insulating layer by via hole conductors which are formed by filling with conductive paste containing at least conductive powder and an organic binder. The process for producing such a multi-layer wiring substrate comprises:

[0116] (a) forming via holes through an uncured or semi-cured insulation sheet which is formed of a mixture containing 50 to 75 vol. % of an inorganic filler and a thermosetting resin,

[0117] (b) filling the via holes with conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet to form via hole conductors,

[0118] (c) forming a metal wiring layer on at least one of the main surfaces of the insulation sheet having the via hole conductors so that the metal wiring layers can be electrically connected to each other through the insulation sheet by the via hole conductors, and

[0119] (d) preparing a predetermined number of the insulation sheet(s) which is obtained in the above (c), and if needed, preparing a predetermined number of other insulation sheet(s); aligning these insulation sheets with one another, and laminating them on one another; and heating and pressing the laminate to thereby cure the thermosetting resins to bond together these insulation sheets so as to obtain an integrated article as the multi-layer wiring substrate.

[0120] The resultant integrated article has, on its expressed surface, the metal wirings which are located at the outermost of the laminate of the insulation sheets, as the metal wirings on the exposed surface of the multi-layer wiring substrate. In this regard, the other insulation sheet used in the step (d) may be the third insulation sheet described above, or any insulation sheet (if needed, having a wiring layer and/or a circuit component and/or via hole conductors) which is necessary to produce a predetermined multi-layer circuit substrate using the insulation sheet obtained in the step (c).

[0121] By using the conductive paste which starts curing at a lower temperature than the melting-starting temperature of the insulation sheet, the curing of the conductive paste has proceeded before the insulation sheet melts and flows, when the insulation sheet is heat-pressed and cured. Therefore, it becomes possible to keep the shapes of the via hole conductors against the flowing of insulation sheet. Under the effect of the via hole conductors' keeping their shapes against flowing of the insulation sheet, the via hole conductors can be sufficiently compressed by the compression of the insulation sheet in the thickness direction, the compression of the metal wirings into the via holes and so on. Thus, the conductive powder of the via hole conductors is compacted into its high density with the result that the electrical connection by the via hole conductors can be obtained with lower resistance and higher reliability. Further, even in a case where the intervals between the via holes formed through the insulation sheet are small, the dislocation of the via holes in association with flowing of the insulation sheet hardly occurs. Therefore, the electrical connection between the metal wiring and the via hole conductors can be obtained with excellent positional accuracy. Further, a failure in connection such as a short circuit of the adjacent wiring circuits or the like is suppressed. Such failure is caused when the conductive paste in the via hole conductors flows out to adhere to the surface area of the insulation sheet between the wiring conductors upon flowing of the insulating layer.

[0122] In one embodiment of the process for producing the circuit component embedded multi-layer wiring substrate of the present invention, a circuit component-embedded multi-layer wiring substrate is produced in which the circuit component is embedded in the insulating layer and the electrical connection between the metal wiring layers is performed through the insulating layer by means of the via hole conductors which are formed by filling the via holes with the conductive paste containing at least the conductive powder and the organic binder. The process comprises:

[0123] (a) forming the via holes through an uncured or semi-cured insulation sheet which is formed of a mixture containing 50 to 75 vol. % of an inorganic filler and a thermosetting resin,

[0124] (b) filling the via holes with the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet so as to form the via hole conductors,

[0125] (c) disposing a metal wiring to which the circuit component has been electrically connected to in advance on at least one of the main surfaces of the insulation sheet having the via hole conductors formed by filling the conductive paste so that the electrical connection between the wiring layers can be performed by the via hole conductors; and simultaneously embedding, in the insulation sheet, the circuit component which is electrically connected to the wiring layer in advance, and

[0126] (d) preparing a predetermined number of such insulation sheet(s) each of which is obtained in the above (c), and if needed, preparing a predetermined number of other insulation sheet(s); aligning these insulation sheets with one another, and laminating them on one another; and heating and pressing the laminate to thereby cure the thermosetting resins to bond together the insulation sheets so as to obtain an integrated article as the multi-layer wiring substrate.

[0127] The resultant integrated article has, on its exposed surface, the metal wirings which are exposed located at the outermost of the laminate of the insulation sheet, as the metal wirings on the exposed surface of the multi-layer wiring substrate. In this regard, the other insulation sheet used in the step (d) may be the third insulation sheet described above, or any insulation sheet (if needed, having a wiring layer and/or a circuit component and/or via hole conductors) which is necessary to produce a predetermined multi-layer circuit substrate using the insulation sheet obtained in the step (c).

[0128] In other embodiment of the process for producing the circuit component embedded multi-layer wiring substrate of the present invention, the circuit component-including multi-layer wiring substrate is produced in which the circuit component is embedded in an insulating layer and the electrical connection between the metal wiring layers is performed through the insulating layer by means of the via hole conductors which are formed by filling with conductive paste containing at least the conductive powder and the organic binder. The process comprises:

[0129] (a) forming the via holes through an uncured or semi-cured insulation sheet which is formed of a mixture containing 50 to 75 vol. % of an inorganic filler and a thermosetting resin,

[0130] (b) filling the via holes with the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet so as to form the via hole conductors,

[0131] (c) disposing a metal wiring on at least one of the main surfaces of the insulation sheet having the via hole conductors formed by filling the conductive paste so that the electrical connection between wiring layers can be performed by the via hole conductors,

[0132] (d) mounting a circuit component on the metal wiring disposed on the insulation-sheet, that is, electrically connecting the circuit component to the metal wiring,

[0133] (e) aligning, with the surface of the insulation sheet having the circuit component mounted thereon, the other insulation sheet which is prepared in the above (a) and (b) and which has the via hole conductors formed by filling the via holes with the conductive paste so that the surface of the metal wiring having the circuit component mounted thereon can be in contact with the via hole conductors of said other separate insulation sheet, and laminating them to obtain a laminate, and

[0134] (f) heating and pressing the laminate to cure the thermosetting resins.

[0135] By using the conductive paste which starts curing at the temperature lower than the melting-starting temperature of the insulation sheet, curing of the conductive paste has proceeded before the insulation sheet flows, when the insulation sheet is heated and pressed and cured. Therefore, the via hole conductors can keep their shapes against not only flowing of the molten insulation sheet but also additional flowing of the insulation sheet which is caused in association with the embedding of the circuit component in the insulation sheet. By this effect of the via hole conductors' keeping their shapes against flowing of the insulation sheet in association with the embedding of the circuit component, the shortest electrical connection is achieved by the via hole conductors filled with the conductive paste on the same insulating layer having the circuit component embedded therein. Further, embedding of the circuit component in the insulation sheet can be carried out without adding a special step to the production steps for producing a wiring substrate including no circuit component.

[0136] In any of the various processes for producing the circuit substrates of the present invention, the thermo-compression bonding and the post-thermo-compression bonding are carried out by two or more times of heating and pressing. For example, the first heating and pressing is done at a temperature higher than the curing temperature of the conductive paste and lower than the melting-starting temperature of the insulation sheet, so that only the via hole conductors formed by filling with the conductive paste are cured; and the insulating layer is cured by the second or any of the subsequent following heating and pressing. Thus, the via hole conductors have been sufficiently cured before the insulation sheet which forms the insulating layer flows, and this further enhances the effect of the via hole conductors' keeping their shapes against flowing of the insulation sheet. This is particularly effective when the via hole conductors and the insulating layer of the insulation sheet having the circuit component embedded therein are cured, because flowing of such an insulation sheet is remarkable.

[0137] Hereinafter, concrete embodiments of the present invention will be explained in more detail with reference to the accompanying drawings.

[0138] (Concrete Embodiment 1 for Carrying Out the Invention)

[0139]FIG. 1 schematically shows a sectional view of a multi-layer wiring substrate (four layers in the shown embodiment) according to one embodiment of the present invention. The wiring substrate (110) comprises insulating layers (100) formed of the insulation sheets, via hole conductors (101) formed by filling with conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet, and wiring layers (102) which provide metal wirings.

[0140] In the present invention, the via hole conductors (101) are formed of the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet. By doing so, the via hole conductors can have the effect to keep their shapes against melting and flowing of the insulation sheet. As a result, the via hole conductors filled with the conductive paste can be compressed, and the conductive powder in the conductive paste can be sufficiently compacted into a high density, so that a multi-layer wiring substrate with lower resistance and highly reliable electrical connection can be obtained. The effect of the via hole conductors' keeping their shapes further provides an effect to improve the positional accuracy of the via hole conductors.

[0141] (Concrete Embodiment 2 for Carrying Out the Invention)

[0142]FIG. 2 schematically shows a sectional view of a circuit component-embedded multi-layer wiring substrate (four layers in the shown embodiment) according to one embodiment of the present invention. The wiring substrate (210) comprises insulating layers (200) formed of the insulation sheets, via hole conductors (201) formed by filling with the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet, and wiring layers (202) which provide the metal wirings, and the insulating layer (200) has embedded circuit components (203) electrically connected to the wiring layer.

[0143] Also, the multi-layer wiring substrate shown in FIG. 2 provides effects which are equivalent to those previously described with reference to FIG. 1. Regarding the insulating layer having the circuit components embedded therein, both of high density mounting of the circuit components by embedding them in the insulating layer and high density electrical connection by the via hole conductors can be achieved at the same time, because the via hole conductors formed by filling with the conductive paste can provide the lower resistant and highly reliable electrical connection.

[0144] (Concrete Embodiment 3 for Carrying Out the Invention)

[0145] A concrete embodiment of the process for producing the multi-layer wiring substrate of the present invention is described with reference to the schematic sectional views shown in FIGS. 3(a) to 3(i).

[0146] An uncured insulation sheet (300) containing an inorganic filler and a thermosetting resin, which forms an insulating layer, is prepared (see FIG. 3(a)), and via holes (301) are formed through the insulation sheet (300) (see FIG. 3(b)). The via holes (301) formed through the insulation sheet (300) are filled with the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet (300) so as to form the via hole conductors (302). Thus, the insulation sheet (310) of the present invention is obtained (see FIG. 3(c)). In parallel to the steps shown in FIGS. 3(a) to 3(c), a transferable wiring pattern-including member (315) is prepared (see FIG. 3(d)). The transferable wiring pattern-including member (315) comprises a release carrier (304) which has a wiring layer (303) including metal wirings to be transferred.

[0147] Next, as shown in FIG. 3(e), the transferable wiring pattern-including member (315) is aligned relative to and laminated on one of the main surfaces of the insulation sheet (310), and subjected to the thermo-compression bonding. After that, the release carrier (304) is removed to thereby complete the thermal transfer of the wiring layer (303). As a result, an insulation sheet (320) which has the wiring layer (303) disposed on one of the main surfaces of the insulation sheet (340) and electrically connected to the via hole conductors (302) (shown in the upper portion of FIG. 3(h)) is obtained.

[0148] As shown in FIG. 3(f), the transferable wiring pattern-including member (315) are aligned relative to and laminated on both main surfaces of the insulation sheet (310) and subjected to the thermo-compression bonding. After that, the release carriers (304) are removed to thereby complete the thermal transfer of the wiring layers (303). As a result, an insulation sheet (330) which has the wiring layers (303) disposed on both main surfaces of the insulation sheet (340) and electrically connected to the via hole conductors (302) (shown in the middle portion of FIG. 3(h)) is obtained.

[0149] Further, as shown in FIG. 3(g), the transferable wiring pattern-including member (315) is aligned relative to and laminated on the other main surface of the insulation sheet (310) and subjected to the thermo-compression bonding. After that, the release carrier (304) is removed to thereby complete the thermal transference of the wiring layer (303). As a result, an insulation sheet (340) which has the wiring layer (303) disposed on the other main surface of the insulation sheet (340) and electrically connected to the via hole conductors (302) (shown in the lower portion of FIG. 3(h)) is obtained.

[0150] The thermo-compression bonding for preparing these insulation sheets is carried out so as to practically cure the conductive paste and, preferably so as to complete the curing of the conductive paste. The curing of the insulation sheets must be selectively carried out so that these insulation sheets can be mutually bonded to each other by the post-thermo-compression bonding which will be carried out later. That is, in a case where the insulation sheets are laminated as shown in FIG. 3(h), it is necessary that the curing of at least one of the adjacent insulation sheets is not completed (including being uncured). Therefore, in a case where one of two adjacent insulation sheets has been completely cured, the curing of the other insulation sheet has not been completed. Both or one of such insulation sheets may be uncured, or may not be completely cured (for example, in a semi-cured state).

[0151] In the embodiment shown in FIG. 3(h), the positions of the wiring layers and the via holes are the same in each of the insulation sheets. However, such arrangement is only an example, for the purpose of simplification. It is clearly understood that the numbers, the positions the materials and the shapes of the wiring layers and the via holes, and may differ in accordance with an intended multi-layer wiring substrate to be produced. Also, similarly, the thickness and the materials of the insulating layers and the insulation sheets may differ.

[0152] Next, the insulation sheets (320, 330 and 330) are aligned relative to and laminated on each other as shown in FIG. 3(h) so that the wiring layers (303) to be electrically connected can contact the via hole conductors (302) when these insulation sheets are subjected to the post-thermo-compression bonding. After that, these insulation sheets are bonded together by the post-thermo-compression bonding. As a result, a multi-layer wiring substrate (350) shown in FIG. 3(i) is obtained. In this post-termo-compression bonding, the insulation sheets are bonded to each other, and the curing of the insulation sheets is completed, and further, the curing of the conductive paste is completed when the conductive sheet has not been sufficiently cured in the previous thermo-compression bonding. The release carrier (304) of the transferable wiring pattern-including member may be removed when each of the insulation sheets is produced as mentioned above. However, in other embodiment, the release carrier which finally locates at the outermost position (the insulation sheet (320) or (340) in the shown embodiment) may be removed after the post-thermo-compression bonding which integrates the insulation sheets.

[0153] By using the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet, the curing of the via hole conductors already has proceeded, and preferably has been completed, before the insulation sheets melt and flow during the post-thermo-compression bonding for obtaining the multi-layer wiring substrate shown in FIG. 3(i). Therefore, the effect of the via hole conductors' keeping their shapes can be obtained, and by this effect, it becomes possible to sufficiently compress the via hole conductors and compact the conductive powder in the conductive paste up to a high density. As a result, the multi-layer wiring substrate can have lower resistance and highly reliable electrical connection between the wiring layers.

[0154] Furthermore, the electrical connection between the wiring layers and the via hole conductors can be realized with improved positional accuracy, because the dislocation of the via hole conductors hardly occurs. Still furthermore, there is provided an effect of suppressing a failure in connection such as a short circuit of the adjacent wiring circuits or the like. Such failure is cause by the conductive paste in the via hole conductors which flows out and adheres to a surface portion of the insulation sheet between the conductive wiring layers.

[0155] Upon the thermally-transferring the wiring layers to the insulation sheets in the course of obtaining the insulation sheets as shown in FIGS. 3(e) to 3(g), such effects as mentioned above become more remarkable by selecting conditions under which melting of the insulation sheets, and preferably melting or curing of the insulation sheets can not be practically caused, while the conductive paste alone in the via hole conductors is being completely cured.

[0156] (Concrete Embodiment 4 for Carrying Out the Invention)

[0157] A concrete embodiment, of the present invention for producing the circuit component-embedded multi-layer wiring substrate is described with reference to the schematic sectional views shown in FIGS. 4(a) to 4(j).

[0158] An uncured insulation sheet (400) containing an inorganic filler and a thermosetting resin, which is to be an insulating layer, is prepared (see FIG. 4(a)), and via holes (401) are formed through the insulation sheet (400) (see FIG. 4(b)). The via holes (401) formed through the insulation sheet are filled with the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet (400) so as to form the via hole conductors (402). Thus, an insulation sheet (410) of the present invention is obtained (see FIG. 4(c)).

[0159] In parallel to the steps shown in FIGS. 4(a) to 4(c), a transferable wiring pattern-including member (415) is prepared (see FIG. 4(d)). The transferable wiring pattern-including member (415) comprises a release carrier (404) which has a wiring layer (403) for providing metal wirings to be transferred. Also, a transferable wiring pattern-including member (416) having circuit components (405) mounted thereon, i.e, electrically connected to the wiring layer (403) is prepared (see FIG. 4(e).

[0160] Next, as shown in FIGS. 4(f) and 4(h), insulation sheets (420) and (440), in which the wiring layers (403) are electrically connected to the via hole conductors (402) by the thermo-compression bonding, are obtained in the same manners as shown in FIGS. 3(e) and 3(g) (shown in the upper and lower portion of FIG. 4(i)).

[0161] As shown in FIG. 4(g), the transferable wiring pattern-including member (415) is aligned relative to and laminated on one of the main surfaces of the insulation sheet (400), and the transferable wiring pattern-including member (416) is aligned relative to and laminated on the other main surface of the insulation sheet (400), and these transferable wiring pattern-including member and the insulation sheet are subjected to the thermo-compression bonding. In this operation, the thermo-compression bonding is carried out so that the circuit components (405) can be embedded in the insulation sheet (400): the thermo-compression bonding is carried out under such a condition that causes the insulation sheet (400) to melt. After that, the release carriers (404) are removed. As a result, there is obtained an insulation sheet (430) which has the wiring layers (403) disposed on both main surfaces of the insulation sheet (440) and electrically connected to the via hole conductors (402), and which has the circuit components (405) having been electrically connected to the wiring layer in advance and then embedded in the insulation sheet (shown in the middle portion of FIG. 4(i)).

[0162] Next, the insulation sheets (420, 430 and 430) are subjected to the post-thermo-compression bonding in the same manner as shown in FIG. 3(h) to obtain a circuit component-including multi-layer wiring substrate (450) as shown in FIG. 4(j).

[0163] By using the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet (400), the curing of the conductive paste in the via holes (401) already has proceeded, and preferably has been practically completed before the insulation sheet (400) flows in association with the embedding of the circuit components (405) in the insulation sheet (400) during the thermo-compression bonding in the step shown in FIG. 4(g). As a result, the shapes of the via holes, i.e., the via hole conductors can be maintained substantially in the same shapes as originally formed through the insulation sheet By this effect of the via hole conductors' maintaining their shapes, it becomes possible to sufficiently compress the via hole conductors and compact the conductive powder in the conductive paste up to a high density. As a result, the multi-layer wiring substrate can have lower resistance and highly reliable interlayer electrical connection. Furthermore, the electrical connection between the wiring layers and the via hole conductors can be realized with improved positional accuracy because the dislocation of the via hole conductors hardly occurs.

[0164] (Concrete Embodiment 5 for Carrying Out the Invention)

[0165] Other concrete embodiment of the process of the present invention for producing the circuit component-including multi-layer wiring substrate is described with reference to the schematic sectional views shown in FIGS. 5(a) to 5(h).

[0166] An uncured insulation sheet (500) containing an inorganic filler and a thermosetting resin, which is to be an insulating layer, is prepared (see FIG. 5(a)), and via holes (501) are formed through the insulation sheet. (500) (see FIG. 5(b)). The via holes (501) formed through the insulation sheet (500) are filled with the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet so as to form the via hole conductors (502). Thus, an insulation sheet (510) of the present invention is obtained (see FIG. 5(c)).

[0167] Next, as shown in FIG. 5(d), metal foils (503) are applied onto both main surfaces of the insulation sheet (510). Then, the metal foils (503) are, for example, chemically etched by the subtractive process as shown in FIG. 5(e) to form wiring layers (504) composed of predetermined metal wirings. In this case, it is preferable that the curing of the conductive paste is allowed to proceed, and more preferably substantially be completed by the thermo-compression bonding. Subsequently, a circuit component (505) is mounted on the metal wiring layer (504) thus formed, so that an insulation sheet (520) having the circuit component on its outer surface is obtained.

[0168] Next, as shown in FIG. 5(f), the insulation sheet (520), an insulation sheet (530) which has via hole conductors (502) and which is to form a layer in which the circuit component (505) is embedded, and a metal foil (540) are prepared. These materials are aligned relative to and laminated on each other so that the insulation sheet (530) is located at the center and so that the metal foil (540) is adjacent to the main surface of the insulation sheet (530) which surface is not opposite to the wiring (504) or the circuit component (505) of the insulation sheet (520).

[0169] After that, as shown in FIG. 5(g), these materials are together subjected to the post-thermo-compression bonding. By this post-thermo-compression bonding, the circuit component (505) is embedded into the insulation sheet (530) (which may be replaced by the insulation sheet (510)), the metal foil (540) is adhered to the insulation sheet (530), the insulation sheets (520, 530) are bonded to each other, and the thermosetting resins in the insulation sheets are completely cured. If the curing of the conductive paste is insufficient before the post-thermo-compression bonding, or if the conductive paste (like, the conductive paste contained in the insulation sheet (530)) is uncured before the post-thermo-compression bonding, the conductive paste is also completely cured by this post-thermo-compression bonding.

[0170] Finally, as shown in FIG. 5(h), the metal foil (540) applied to the insulation sheet is, for example, chemically etched by the subtractive process to form predetermined metal wirings. Thus, a circuit component-including multi-layer wiring substrate (550) is obtained.

[0171] By using the conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet, the shapes of the via hole conductors formed by filling with the conductive paste can be maintained against flowing of the insulation sheet in association with embedding of the circuit component, and thus, effects equivalent to those obtained by the previous embodiment 4 can be obtained.

EXAMPLES Example 1

[0172] A four-layer wiring substrate, as shown in FIG. 1, comprising via hole conductors formed by filling with conductive paste was produced according to a process similar to the process previously described with reference to FIG. 3. For the ease of understanding, reference numerals which are the same as those denoting the elements shown in FIG. 3 are also used herein.

[0173] First, a transferable wiring pattern-including member (315) for use in forming a wiring layer (303) having predetermined wirings was prepared as follows. An electrolytic copper foil with a thickness of 70 μm was prepared as a first metal layer which functions as a substrate of a release carrier. A Ni plating layer with a thickness of 100 to 200 nm as a release layer was formed on the first metal layer to provide a release carrier (304). Then, a second metal layer of copper with a thickness of 9 μm was formed on the Ni plating layer by the electrolytic plating process to obtain a three-layer structure comprising the first metal layer, the release layer and the second metal layer which was to form the metal wirings.

[0174] Next, a wiring layer was formed by the photolithographic process: a dry film resist was applied on the second metal layer, and a portion of the resist corresponding to the wiring pattern portion of the second metal layer was exposed and developed followed by the subtractive process such as chemical etching to form the wiring layer (303); and finally, the exposed portion of the dry film resist used as the mask was removed. Thus, the intended transferable wiring pattern-including member (315) was obtained (see FIG. 3(d)).

[0175] Next, an insulation sheet (300), with a thickness of 200 μm, containing an uncured thermosetting resin was prepared (see FIG. 3(a)). Then, a predetermined number of via holes (301) with a diameter of 200 μm were formed through the insulation sheet (see FIG. 3(b)), and the via holes (301) were filled with conductive paste to form via hole conductors (302). Thus, an insulation sheet (310) having the via hole conductors (302) formed therein was prepared (see FIG. 3(c)).

[0176] The above transferable wiring pattern-including members (315) were aligned relative to and laminated on both main surfaces of the insulation sheet (310) (see FIG. 3(f)), and heated and pressed at 200° C. under a pressure of 75 kg/cm² for 1.5 hours. By doing so, the transferable wiring pattern-including members (315) and the insulation sheet (310) were thermally compressed and bonded to each other, so that the wiring layers (303) were disposed on the insulation sheet and electrically connected to the via hole conductors (302). After that, the release carriers (304) of the transferable wiring pattern-including members were removed to obtain the intended insulation sheet (330) having the wiring layers on both main surfaces thereof.

[0177] Next, the insulation sheets (310) having the via hole conductors formed therein were aligned relative to and laminated on both main surfaces of the insulation sheet (330), and further, the transferable wiring pattern-including members (315) were aligned relative to and laminated on the main surfaces of the insulation sheets (310) which did not confront the insulation sheet (330). In this regard, the process shown in FIG. 3 differs from the present example in that the insulation sheets (320) and (340) having the wiring layers thereon in advance were used instead of the insulation sheets (310) and the transferable wiring pattern-including members (315).

[0178] These insulation sheets (310, 310) and (330) and the two wiring pattern-including members (315, 315) were heated and pressed at 200° C. under a pressure of 75 kg/cm² for 1.5 hours for the post-thermal-compression bonding so as to bond them together. By this post-thermo-compression bonding, the wiring layers (303) and the insulation sheets (310) were bonded to each other, the insulation sheets (310 and 330) were bonded to each other, and the thermosetting resins in the insulation sheets and the conductive paste were cured. After that, the release carriers (304) of the transferable wiring pattern-including members (315) were peeled off to obtain the intended four-layer wiring substrate shown in FIG. 1.

[0179] In this example, three types of the conductive paste having the following compositions and curing-starting temperatures of 90° C., 100° C. and 120° C., respectively were prepared, and by using the respective conductive paste, substrate (1) (the curing-starting temperature of 90° C.), substrate (2) (the curing-starting temperature of 100° C.) and substrate (3) (the curing-starting temperature of 120° C.) were obtained. Hereinafter, the compositions of the material used for making the insulation sheets and the process of making the same will be described.

[0180] (Composition of Material Used for Production of Insulation Sheet) Al₂O₃ (AS-40 with a particle size of 12 μm, 90 wt. % manufactured by Showa Denko K.K.): Liquid thermosetting epoxy resin (EF-450, 9.5 wt. % manufactured by Japan Rec Co., Ltd.) Carbon black (manufactured by Toyo 0.2 wt. % Carbon Co., Inc.) Coupling agent (titanate type, 46B, 0.3 wt. % manufactured by Ajinomoto Co., Inc.)

[0181] The respective components as above were weighed according to the above composition to obtain a mixture, and methyl ethyl ketone as a viscosity-adjusting solvent was added to the mixture so that a slurry viscosity of the mixture was adjusted to about 20 Pa.s. Next, alumina balls were added to the mixture, and they were rotated at 500 rpm in a pot for 48 hours to prepare a slurry as a raw material for the insulation sheet. Then, the slurry was applied in a film form with a doctor blade and dried. Thus, the insulation sheet was obtained.

[0182] Next, the melting-starting temperature of the insulation sheet was measured. Upon the measurement, a temperature vs viscosity characteristic of the insulation sheet was measured using the apparatus described before. The melting-starting temperature of the insulation sheet was determined as follows: the insulation sheet was heated from a room temperature, and a temperature at which the viscosity of the insulation sheet at 50° C. decreased to one-fiftieth ({fraction (1/50)}) was defined as the melting-starting temperature of the sheet. The melting-starting temperature of the insulation sheet prepared above was 100° C. In this regard, the temperature at which the viscosity of the insulation sheet at a room temperature decreased to one-hundredth ({fraction (1/100)}) was substantially the same as the temperature at which the viscosity of the insulation sheet at 50° C. decreased to one-fiftieth.

[0183] (Conductive Paste) Spherical copper particles (with a particle size of 5 μm, 85 wt. % manufactured by Mitsui Milling and Smelting Co., Ltd.) Bisphenol F type thermosetting epoxy resin (Epicoat 807, 7 wt. % manufactured by Yuka Shell Epoxy K.K.) Novolak type phenol resin (BRG558, manufactured by 3 wt. % Showa Highpolymer Co., Ltd.) Butyl carbitol (Japan Alcohol Trading Co., Ltd.) 5 wt. %

[0184] To 100 parts by weight of the total of the above components was added 1 part by weight, 0.7 parts by weight or 0.5 parts by weight of 2-ethyl-4-methylimidazole as a curing accelerator, and the mixture was kneaded with a three-roll mill to form paste.

[0185] Using the above described process and the apparatus, the curing-starting temperatures of the three kinds of the conductive paste were measured by the thermal analysis by means of DTA (differential thermal analysis). As a result, the curing-starting temperature of the conductive paste (1 part by weight of 2-ethyl-4-methylimidazole added) was 90° C.; the curing-starting temperature of the conductive paste (0.7 parts by weight of 2-ethyl-4-methylimidazole added) was 100° C.; and the curing-starting temperature of the conductive paste (0.5 parts by weight of 2-ethyl-4-methylimidazole added) was 120° C.

[0186] The wiring substrates as produced above were evaluated as follows:

[0187] The total resistance value of 300 via holes, electrically connected in series, of each of the above produced wiring substrates was measured immediately after the production of the wiring substrate (the initial electrical resistance value), after the same wiring substrate was left to stand under an atmosphere of 85° C. and a relative humidity of 85% for one week, after the same wiring substrate firstly passed through a reflow furnace at the highest temperature of 230° C. after the above standing, after the same wiring substrate passed through the reflow furnace 5 times, and after the same wiring substrate passed through the reflow furnace 10 times, respectively. The results of the measurement of the resistance values are shown in Table 1. TABLE 1 Resistance (Ω) in Reliability Test Passing Passing Passing Resistance through through through after After reflow reflow reflow production absorbing furnace furnace furnace (Ω) moisture (one time) (5 times) (10 times) Wiring 7.52 7.75 8.08 8.07 8.06 substrate 1 (the present invention) Wiring 18.02 23.50 125.30 121.90 118.70 substrate 2 (Com. Ex.) Wiring 32.10 50.20 250.50 247.40 243.30 substrate 3 (Com. Ex.)

[0188] As understood from Table 1, the wiring substrate 1, which was produced using the conductive paste which started curing at 90° C. lower than the melting-starting temperature of 100° C. of the insulation sheet, showed the lower resistance in any of the tests, as compared with wiring substrates 2 and 3. This means that the electrical connection in the wiring substrate 1 of the present invention is superior in the electrical resistance and reliability.

Example 2

[0189] A circuit component-including wiring substrate of the present invention, which had a circuit component embedded in its insulating layer, was produced. Since the process of producing the wiring substrate is partially similar to the process described with reference to FIG. 4, reference numerals which are the same as those denoting the elements shown in FIG. 4 are herein used for the ease of understanding.

[0190] A transferable wiring pattern-including member (415) was prepared in the same manner as in Example 1. Further, a semiconductor device with a thickness of 300 μm (10 mm (lengthwise)×10 mm (widthwise)) was bonded on the metal wiring formed on the transferable wiring pattern-including member by the flip chip bonding using an electrically conductive adhesive. Thus, a transferable wiring pattern-including member (416) having the semiconductor device (405) as a circuit component was prepared.

[0191] Using the same materials as in Example 1, an insulation sheet (400) with a thickness 600 μm was prepared, and a predetermined number of via holes (401) with a diameter of 400 μm were formed through the insulation sheet and filled with the same conductive paste as in Example 1, to form via hole conductors (402). Thus, an insulation sheet (410) having the via hole conductors (402) formed therein was prepared. In this regard, the via holes were formed 500 μm away from the periphery of a region where the semiconductor device is to be embedded, that is, the outer peripheral portion of the semiconductor device (i.e., the four sides of the device).

[0192] Next, the insulation sheet (410), the transferable wiring pattern-including member (415) having the wiring layer (403) formed thereon, and the transferable wiring pattern-including member (416) having the semiconductor device (405) mounted thereon were aligned relative to and laminated on each other, and subjected to heating and is pressing at 175° C. under a pressure of 10 kg/cm² for 1.5 hours. Thus, the semiconductor device (405) on the transferable wiring pattern-including member was embedded in the insulation sheet (410), and the insulation sheet and the conductive paste were cured.

[0193] After that, the transfer carriers (404) of the transferable wiring pattern-including member were peeled off to transfer the wiring layers onto the insulation sheet. Thus, a both-sided wiring substrate having the semiconductor device (405) embedded in the insulating layer (corresponding to the insulation sheet (430) which has been completely cured) was obtained. Using the same conductive paste as in Example 1 (the three types of the conductive paste having curing-starting temperatures 90° C., 100° C. and 120° C., respectively), circuit component-embedded wiring substrate 4 (the curing-starting temperature of 90° C.), circuit component-embedded wiring substrate 5 (the curing-starting temperature of 100° C.), and circuit component-embedded wiring substrate 6 (the curing-starting temperature of 120° C.) were produced. The resultant wiring substrates were evaluated in the same manner as in Example 1. The results are shown in Table 2. TABLE 2 Resistance (Ω) in Reliability Test Passing Passing Passing Resistance through through through after After reflow reflow reflow production absorbing furnace furnace furnace (Ω) moisture (one time) (5 times) (10 times) Circuit 2.02 2.25 2.58 2.57 2.52 component- embedded wiring substrate 4 (the present invention) Circuit 8.01 13.10 98.90 95.20 92.30 component- embedded wiring substrate 5 (Com. Ex.) Circuit X X X X X component- embedded wiring substrate 6 (Com. Ex.)

[0194] As seen from Table 2, wiring substrate 4 produced using the conductive paste which started curing at 90° C. lower than the melting-starting temperature of 100° C. of the insulation sheet, showed the lower resistance in any of the tests, as compared with wiring substrates 5 and 6. Thus, it is found that the electrical connection in wiring substrate 4 of the present invention was superior in the electrical resistance and reliability. The mark “X” seen in the table means that electrical connection was impossible because of the dislocation of the via holes.

[0195] Effect of the Invention

[0196] According to the present invention, in the production of the multi-layer wiring substrate and the circuit component-embedded wiring substrate, the interlayer electrical connection is performed by the via hole conductors formed by filling with conductive paste which starts curing at a temperature lower than the melting-starting temperature of the insulation sheet. By doing so, the via hole conductors can maintain their shapes against flowing of the melted insulation sheet and flowing of the insulation sheet in association with embedding of the circuit component in the insulation sheet. Thus, the via hole conductors can be sufficiently compressed, and the conductive powder in the conductive paste can be compacted up to the high density. Therefore, the multi-layer wiring substrate and the circuit component-including multi-layer wiring substrate can have the lower resistance and the higher reliability. 

What is claimed is:
 1. A multi-layer wiring substrate comprising at least one insulating layer, wiring layers disposed on both sides of the insulating layer, and via hole conductors which are so located in via holes penetrating the insulating layer as to be electrically connected to the wiring layers, wherein the via hole conductors are formed of a conductive paste having a curing-starting temperature lower than a melting-starting temperature of an insulation sheet which is to form the insulating layer.
 2. The multi-layer wiring substrate according to claim 1, wherein the curing-starting temperature of the conductive paste is 50 to 150°.
 3. The multi-layer wiring substrate according to claim 1, wherein the conductive paste comprises conductive powder and an organic binder, and a weight ratio of the conductive powder to the organic binder is 85 to 95:5 to
 15. 4. The multi-layer wiring substrate according to claim 3, wherein the conductive powder is at least one selected from the group consisting of metal powder of gold, silver, palladium, copper, nickel, tin or lead, powder of an alloy of any combination of said metals, and powder whose individual particle has a core of at least one selected from said metals and said alloys, and a coating of a metal or an alloy selected from said metals and said alloys other than the metal or the alloy of the core.
 5. The multi-layer wiring substrate according to claim 3, wherein the organic binder contains a thermosetting resin which is at least one selected from the group consisting of epoxy resins, phenol resins, cyanate resins, and polyphenylene ether resins.
 6. The multi-layer wiring substrate according to claim 3, wherein the organic binder contains a curing catalyst or a curing accelerator which comprises at least one compound selected from the group consisting of amine compounds, imdazoles, imidazole derivatives, and onium salts.
 7. The multi-layer wiring substrate according to claim 1, wherein the insulation sheet contains a thermosetting resin which is at least one selected from the group consisting of epoxy resins, phenol resins and cyanate resins.
 8. The multi-layer wiring substrate according to claim 1, wherein the insulation sheet contains an inorganic filler in an amount of 50 to 70 vol. % based on the volume of the insulation sheet.
 9. The multi-layer wiring substrate according to claim 8, wherein the inorganic filler comprises at least one selected from the group consisting of Al₂O₃, SiO₂, MgO, BN and AlN.
 9. The multi-layer wiring substrate according to claim 1, wherein a circuit component electrically connected to at least one wiring layer and embedded in the insulating layer is comprised.
 10. An insulation sheet for use in the production of a wiring substrate, said insulation sheet comprising, as via hole conductors, conductive paste which is filled in through via holes formed through the insulation sheet, and which has a curing-starting temperature lower than the melting-starting temperature of the insulation sheet.
 11. The insulation sheet according to claim 10, wherein the curing-starting temperature of the conductive paste is 50 to 150° C.
 12. The insulation sheet according to claim 10, wherein the conductive paste contains conductive powder and an organic binder, and a weight ratio of the conductive powder to the organic binder is 85 to 95:5 to
 15. 13. The insulation sheet according to claim 12, wherein the conductive powder is at least one selected from the group consisting of metal powder of gold, silver, palladium, copper, nickel, tin or lead, powder of an alloy of any combination of said metals, and powder whose individual particle has a core of at least one selected from said metals and said alloys, and a coating of a metal or an alloy selected from said metals and said alloys other than the metal or the alloy of the core.
 14. The insulation sheet according to claim 12, wherein the organic binder contains a thermosetting resin which is at least one selected from the group consisting of epoxy resins, phenol resins, cyanate resins, and polyphenylene ether resins.
 15. The insulation sheet according to claim 12, wherein the organic binder contains a curing catalyst or a curing accelerator which comprises at least one compound selected from the group consisting of amine compounds, imdazoles, imidazole derivatives, and onium salts.
 16. The insulation sheet according to claim 10, which contains a thermosetting resin being at least one selected from the group consisting of epoxy resins, phenol resins and cyanate resins.
 17. The insulation sheet according to claim 10, which contains an inorganic filler in an amount of 50 to 70 vol. % based on the volume of the insulation sheet.
 18. The insulation sheet according to claim 17, wherein the inorganic filler comprises at least one selected from the group consisting of Al₂O₃, SiO₂, MgO, BN and AlN.
 19. A process for preparing an insulation sheet, which process comprises forming via holes through an insulation sheet, and filling the via holes with conductive paste which has a curing-starting temperature lower than the melting-starting temperature of the insulation sheet.
 20. The process according to claim 19, wherein the curing-starting temperature of the conductive paste is 50 to 150° C.
 21. The process according to claim 19, wherein the conductive paste contains conductive powder and an organic binder, and the weight ratio of the conductive powder to the organic binder is 85 to 95:5 to
 15. 22. The process according to claim 21, wherein the conductive powder is at least one selected from the group consisting of metal powder of gold, silver, palladium, copper, nickel, tin or lead, powder of an alloy of any combination of said metals, and powder whose individual particle has a core of at least one selected from said metals and said alloys, and a coating of a metal or an alloy selected from said metals and said alloys other than the metal or the alloy of the core.
 23. The process according to claim 21, wherein the organic binder contains a thermosetting resin which is at least one selected from the group consisting of epoxy resins, phenol resins, cyanate resins, and polyphenylene ether resins.
 24. The process according to claim 21, wherein the organic binder contains a curing catalyst or a curing accelerator which comprises at least one compound selected from the group consisting of amine compounds, imdazoles, imidazole derivatives, and onium salts.
 25. The process according to claim 19, wherein the insulation sheet contains a thermosetting resin which is at least one selected from the group consisting of epoxy resins, phenol resins and cyanate resins.
 26. The process according to claim 19, wherein the insulation sheet contains an inorganic filler in an amount of 50 to 70 vol. % based on the volume of the insulation sheet.
 27. The process according to claim 26, wherein the inorganic filler comprises at least one selected from the group consisting of Al₂O₃, SiO₂, MgO, BN and AlN.
 28. A process for producing a wiring substrate, comprising: (a) preparing the insulation sheet as defined in claim 10, and (b) disposing a wiring layer on at least one of main surfaces of the insulation sheet to thereby electrically connect the via hole conductors to the wiring layer.
 29. The process according to claim 28, wherein the electrical connection of the wiring layer to the via hole conductors by disposing the wiring layer is done by thermo-compression bonding.
 30. The process according to claim 29, wherein the thermo-compression bonding is done by thermal transfer of the wiring layer to the insulation sheet.
 31. The process according to claim 29, wherein the thermo-compression bonding is done by heating and bonding a metal foil to the insulation sheet, if needed, under a pressure, and then, chemically etching the metal foil to form a wiring layer.
 32. The process according to claim 29, wherein the thermo-compression bonding is done in a plurality of steps, and the conductive paste is substantially cured without substantially melting the insulation sheet in a first thermo-compression bonding step, and the curing of the insulation sheet is allowed to proceed and completed in at least one following thermo-compression bonding step.
 33. The process according to claim 29, wherein the wiring substrate is a single-sided or both-sided wiring substrate, and wiring layers are disposed on both main surfaces of the insulation sheet, and (c) the curing of the insulation sheet is completed by carrying out post-thermo-compression bonding after the thermo-compression bonding to thereby obtain the wiring substrate.
 34. The process according to claim 29, wherein the wiring substrate is a multi-layer wiring substrate, and said process includes (d) laminating at least one other insulation sheet on at least one of the main surfaces of the insulation sheet obtained after the thermo-compression bonding, wherein (e) this lamination is carried out by (e-1) aligning the insulation sheet relative to said other insulation sheet which is adjacent thereto so that predetermined regions of the wiring layer of the insulation sheet can be electrically connected to via hole conductors of said other insulation sheet which is adjacent thereto in a predetermined manner by post-thermo-compression bonding which is to be carried out subsequently, or so that the via hole conductors of the insulation sheet can be electrically connected to predetermined regions of wiring layer of said other insulation sheet which is adjacent thereto in a predetermined manner, by post-thermo-compression bonding which is to be carried out subsequently, and (e-2) aligning the rest of said other insulation sheets so that the rest of said other insulation sheets can be electrically connected to each other in a predetermined manner by the post-thermo-compression bonding which is to be carried out subsequently; and (f) subjecting the lamination of the insulation sheets to the post-thermo-compression bonding to obtain the multi-layer wiring substrate.
 35. The process according to claim 34, wherein the thermo-compression bonding does not lead to complete curing or no curing, and the post-thermo-compression bonding finishes curing.
 36. The process according to claim 34, wherein the post-thermo-compression bonding is carried out in a plurality of steps, and, if the curing of the conductive paste is not completed by the thermo-compression bonding, the conductive paste is substantially cured without substantially melting the insulation sheets, in the first post-thermo-compression bonding step, and the curing of the insulation sheets is completed in at least one of the following post-thermo-compression bonding steps.
 37. The process according to claim 30, wherein a circuit component is electrically connected to the wiring layer to be thermal transferred, and the thermal transfer is carried out so that the wiring layer is exposed on the main surface of the insulation sheet, and that the circuit component is embedded in the insulation sheet.
 38. The process according to claim 34, wherein the insulation sheet obtained after the thermo-compression bonding has a circuit component which is electrically connected to the wiring layer thereof in advance and embedded in the insulation sheet.
 39. The process according to claim 34, wherein the insulation sheet obtained after the thermo-compression bonding has a circuit component which is electrically connected to the wiring layer thereof and located outside the insulation sheet.
 40. The multi-layer wiring substrate according to claim 1, wherein the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet by at least 5° C.
 41. The insulation sheet according to claim 10, wherein the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet by at least 5° C.
 42. The process according to claim 19, wherein the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet by at least 5° C.
 43. The process according to claim 28, wherein the curing-starting temperature of the conductive paste is lower than the melting-starting temperature of the insulation sheet by at least 5° C. 